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Merlin, also known as Myrdin, and originally known as Moros, is a Lantean from the city of Atlantus. He was the High Councilor, placing him in charge of the city and, potentially, of much of the Lantean empire. Little of him is known, except that, like many others, he fled through the Stargate to Earth during the Wraith siege (also known as the Hundred Year Siege).
During this time, he created a supply cache in England, where he and Arthur Pendragon (known as "King Arthur" or "The Once and Future King", the son of Uther Pendragon, both of whom wielded The Sword in the Stone/Excalibur), before leaving Earth for other strongholds, where he sought weapons capable of defeating the Ori. As he was in danger of dying, he placed himself in stasis (or was placed in stasis, rather), where he waited for hundreds/thousands of years, to be revived by Stargate Command Team 1. Shortly after his revival, his physical body passed away, after his mind was downloaded into a Repository of Knowledge, where it was then downloaded into and temporarily merged with the psyche of Doctor Daniel Jackson, who used it to fend off the Orici to cover SG-1's retreat, and later to finish the weapon. After completing the weapon, the downloaded version disappeared from Doctor Jackson, rendering Merlin effectively dead.
It is unknown at what point he ascended, and, also, at what point he descended back into the mortal plane. Legend has it, however, that he aged backwards through time, which may indicate some level of time-travel capabilities (he was familiar with Janus, who created a time machine).